Underfloor heating

[MahBumble]

I'm stuck trying to model underfloor heating. Are there any tools in Apache to handle it?

I'm sure this has been brought up before, but I cannot find a link to the old topic. Apologies.

Thanks.

[JosephG]

There are many ways of modeling underfloor heating, it all depends on what question is supposed to be answered by creating a model of underfloor heating. So, what are you trying to find out from modeling it?

[MahBumble]

I'd like to explore the lag in the system between the heating switching on and the space getting to temperature, and to calculate the heat loads required for a setpoint temperature. I'm modelling a small dwelling.
Thanks

[JosephG]

Okay, you can do that using Apache Systems. A few tips:

1. Make sure you change the heating plant radiant fraction from default

2. Make sure your building fabric is accurate.

3. You may need to add to the internal side of the building fabric an extra amount of material to account for the home furnishings' thermal mass. Do not use the furniture mass factor because your heating plant has a significant radiant component, while the furniture mass factor merely amplifies the heat contained in the air.

You might get better results using ApacheHVAC from a system dynamics and building energy point of view, but the above should suffice to understand the time lag effects.

[MahBumble]

Thanks for the swift reply - three weeks ago! Can you offer any advice on calculating the radiant fraction of the slab? Have been using 0.5 when fiddling with test models, but looking for a more accurate figure.

[JosephG]

That's an entirely empirical question. Radiant fractions vary from one installation to the next. I'd use whatever guidance is issued by the HVAC body in your country or if the manufacturer is willing to commit to one.

[MahBumble]

Thanks for the help. Time to dig into CIBSE Guide B then I suppose!